1. Field of the Invention
The invention relates to an image processing system which receives asynchronous analog video output signals from a plurality of independently operating video cameras and simultaneously diplays the image obtained from each camera, dimensionally reduced by a factor of two, on a non-overlapping basis on one video monitor.
2. Description of the Prior Art
In video surveillance applications, one often needs to monitor areas situated at different locations. Illustrative applications include remote monitoring of: separate gaming tables at a casino, each entrance/exit at an apartment complex or office building, or stations along a production line or in a dangerous piece of equipment, e.g. a nuclear reactor. For these situations, a separate video camera is stationed at each location and positioned to repetitively scan a particular scene. The analog video output from all the cameras is then routed to a central location and displayed on remote video monitors located there. To minimize the number of separate remote monitors, one monitor is often used to display all the scanned images.
When, as in many surveillance applications, the scanned images change slowly, an operator does not have to continuously monitor each scanned area. Hence, a simple switch can be incorporated into the surveillance system at the central location to allow the operator to select which camera output is to be displayed at any instant on the remote monitor. Through experience, the operator will recognize which areas require more surveillance and which require less and will thereby change the setting of the switch to successively display all the scanned areas according to their respective needs for repetitive surveillance.
By contrast, so-called high surveillance applications impose very stringent surveillance requirements. Here, all the scanned images must be displayed continuously and no discretion must be given to an operator to select only one of the images for display at any one time and not to display the others. Therefore, in these high surveillance applications, a need has arisen to continuously and simultaneously display multiple scanned images on one remote video monitor.
In an optimum high surveillance system, all cameras should operate totally asynchronously with respect to each other. Furthermore, the video output from each camera should be digitized into at least 6 bits (for 64 gray levels) and then applied to a separate section of a large capacity frame store random access memory (RAM) which contains the digital representation of a single composite image. From the frame store memory, the composite image, in which all the component images are located in different nonoverlapping areas, is then sent, via an analog-to-digital (A/D) converter, to the remote monitor for display.
Unfortunately, the art has not surmounted one major technical hurdle, involving a speed-cost tradeoff inherent in frame store memory, which has effectively prevented a low-cost multiple image display surveillance system from being developed. Specifically, in order to display an image on a monitor with adequate resolution, the picture elements (pixels) comprising that image must be displayed at a high rate (approximately 7.5 MHz or 135 nano-seconds/pixel). To display all the video data in a frame store memory, the total number of writes to that memory must equal or be less than the total number of reads from that memory. However, if there are fewer writes than reads, then the display update rate will be compromised. Hence, the frame store memory must be implemented from RAM having a maximum memory cycle time of approximately 68 nano-seconds. This speed is four times faster than that of commonly available large capacity N-type metal-oxide semiconductor (NMOS))integrated circuit (IC) dynamic RAM chips. Moreover, RAM memory ICs having a 68 nano-second cycle time currently exist only in a few integrated circuit technologies (such as emitter coupled logic or bipolar), and those ICs which possess that capability disadvantageously consume an excessive amount of power and only have small memory sizes. Unfortunately, these small memory ICs are quite expensive. The high cost and excessively high power consumption prohibit the use of these relatively "fast" memory ICs in a high capacity frame store memory and has therefore precluded the use of a high capacity frame store memory in low cost multiple image display surveillance systems. In particular, one presently available multiple image display system, which utilizes a frame store memory and has the capability to simultaneously display 16 separate scanned images, unfortunately costs in excess of $25,000.00.
To circumvent this speed-cost limitation, a number of stop-gap solutions, aimed at eliminating the need for a high capacity frame store memory, appear in the art; however, as discussed below, all of these prior art solutions possess serious drawbacks of one form or another.
For example, one system, which sees wide use in the art, relies on using a first video camera as a master camera and a second video camera as a blockdriven synchronized slave camera. Operation of the slave camera is fully synchronized, both horizontally and vertically, with that of the master camera. In one mode of operation, the system can selectively display the scanned image produced by either camera, or alternatively, in another mode, can superimpose the images on top of each other to create a "picture-within-a-picture" display. In the latter mode, a region (window) in the image produced by the master camera (hereinafter referred to as the background image) is electrically defined. Part of the scanned image produced by the slave camera (the foreground image) is then written into that window. Unfortunately, this prior art system possesses one very serious drawback: the portion of the background image, which is covered by the foreground image, is not displayed at all. Hence, any activity occurring in that portion of the background image can not be seen by the operator. This deficiency renders this prior art system completely unsuitable for use in high surveillance and many other applications.
Another prior art system, which displays four separate images simultaneously as a composite image on one monitor, relies on synchronizing a group of four video cameras together, both horizontally and vertically, and operating each camera to scan its image four times faster than normal video scan rates (i.e. twice as fast vertically, and twice as fast horizontally). The composite image consists of four separate quadrants in which each quadrant contains the scanned image produced by a different one of the cameras. Control electronics, located external to the cameras, select the particular camera in real-time that is to provide picture information for any one portion of the composite image. Once the camera has been selected, it is then instructed to scan its image area and provide picture information during the time that portion is being displayed on the remote monitor. Since the vidicon tube within each camera must operate well in excess of its normal vertical and horizontal scan rates, this prior art system unfortunately produces a unclear picture. In particular, to produce a video signal having an optimum signal-to-noise (S/N) ratio, a vidicon must be operated at or near normal scan rates. Operation at significantly higher scan rates does not provide the vidicon with sufficient time, during the scanning of each picture element, to completely react to the available light. Consequently, the resulting image appears quite grainy. Moreover, synchronizable high scan rate cameras are not common in the industry and are thus disadvantageously quite expensive. In addition, since non-standard control (select) signals must be run to each camera, installation wiring becomes complicated.
Another prior art system relies on routing the video output from each one of four cameras to a separate monitor. All four monitors are then positioned close together inside a light-tight enclosure along with a fifth camera which scans the images produced on all the monitors. This fifth camera produces a composite image that contains the images generated on all four monitors. While this solution is quite simple, the composite image contains geometric distortion and usually suffers from poor contrast and brightness. Specifically, each of the four displayed images, by virtue of being displayed on a cathode ray tube (CRT), has rounded corners and artificial curvature. When each of the four displayed images is scanned by a camera and then displayed on another CRT, additional rounding and artificial curvature, i.e. geometric distortion, is introduced into the composite image. In addition, an automatic gain control circuit (AGC) exists in each camera to regulate the gain of that camera in response to the average value of the light intensity occurring over the entire image scanned by that camera. Hence, if any one of the four displayed images, in this prior art system, is unusually bright, then the AGC associated with the fifth camera will reduce its gain and unnecessarily darken the remaining three displayed images. Consequently, apart from the geometric distortion, the brightness of the four images in the composite image will interact, and hence these images may be displayed with insufficient brightness and contrast.
Thus, a need has existed for quite some time in the art to provide an effective, low-cost multiple image display system, for use in high surveillance applications.